As an engine valve operating system, a variable valve timing mechanism capable of varying valve timing of intake and/or exhaust valves depending on an engine operating condition is generally known. For instance, as disclosed in Patent document 1, the variable valve timing mechanism has a first rotor that rotates in synchronism with rotation of a crankshaft of an engine and a second rotor that rotates together with a camshaft of the engine, the second rotor also configured to be rotatable relatively to the first rotor. Valve timing of the intake/exhaust valve, opened and closed by the camshaft, is variable by changing relative rotation positions of both of the rotors by means of an actuator.
Additionally, in the Patent document 1, also provided is an intermediate lock mechanism capable of restricting the relative rotation positions of both of the rotors (i.e., a rotation phase), corresponding to a valve timing value, at a predetermined intermediate lock position. The intermediate lock mechanism is configured to be able to restrict the relative rotation positions of both of the rotors at the predetermined intermediate lock position by bringing a lock piece attached to the one rotor into engagement with an engaged groove formed in the other rotor. For instance, when an engine stop request is detected, the relative rotation positions of both of the rotors are restricted or fixed at the intermediate lock position suited for starting the engine, and whereby it is possible to smoothly perform next engine starting.